1. Field of the Invention
The present invention relates to a helical antenna and an in-vehicle antenna including the helical antenna.
2. Description of Related Art
Conventionally, a helical antenna is widely-used as a linear antenna having good circular polarization characteristics. When such a helical antenna is used on its own, it is difficult to control directivity of an antenna beam. Accordingly, in a publication of JP-A-8-78946, an array structure is employed, in which helical antennas that form beams having an identical shape are arranged on a planar ground plane, in order to control directivity of a helical antenna whose one turn corresponds to one wavelength (i.e., one turn of the helical antenna measures one wavelength in circumferential length). In JP-A-8-78946, the directivity is controlled by making the beams formed by the helical antennas having the array structure interfere with each other.
However, in the case of the antenna having an array structure as in JP-A-8-78946, the helical antennas need to be arranged at intervals of a half of a wavelength λ, i.e., λ/2 in order to control the directivity with the shape of the antenna beam maintained. As a result, the helical antennas at least need to be arranged at intervals of λ/2, so that there is a limit to downsizing of the entire helical antenna.